Use of catheters requires that several important prerequisites are fulfilled. The catheter must be kept sterile during insertion in a blood vessel. This means that during insertio the catheter cannot be exposed to the doctor's hands for example. Further it is very important that the free end of the catheter is positioned at the right place in the blood vessel and in order to do this one must know in advance how far in the catheter should be inserted. Until now the most common method has been using X-rays, but this method is complicated and can only be used if there is enough time available, which is not the case if the catheter is inserted during a major operation. For accidents and outside hospitals X-ray checking is entirely impossible and furthermore an incorrect or too deeply inserted catheter can result in serious complications such as puncture of the blood vessel or the catheter's entering the heart.
Investigations have been made to determine the average distance between different insertion points and the positions wheere the catheter point is desirably placed. These investigations have shown that for grown-ups the length variations are rather large, for example the distance between the insertion point in the right elbow vein and the right auricle of the heart is 48.8+-6.0 cm (2 S.D) for women and 53.0+-6.0 (2 S.D) for men. It is important that the point of the catheter arrives at the right position, and this is one of the reasons that a careful measurement of the insertion length of the catheter must be made.
It is known that one can make a printed scale on the catheter itself, but because the catheter has a diameter of about 1-2 mm it is very difficult to read. A printed scale on the outside of the catheter is furthermore quite inappropriate because any form of marking or ridging of the catheter itself increases the risk for causing thromboses as a result of accumulation of blood platelets at or in the roughnesses in the surface of the catheter. Further the printing ink can be partly or completely dissolved by the blood or rubbed off by friction against the blood vessel walls.
According to another proposal the catheter is wound up on a drum and by turning it in a circular holder the catheter can be fed out. The drum can be equipped with an index so that one can keep track of how many turns the drum has made and with knowledge of the circumference of the drum (in the present case 5 inches=127 mm) one can make a rough estimation of how much catheter has been fed out. However, one does not get a continuous length indication and one must continually calculate parts of turns into centimeters. The method is also only usable if the catheter is wound up on a drum.
Also known is a packaging of a catheter in the form of a plastic bag which is detachably affixed to a connecting piece, to which a cannula can also be affixed. The plastic bag allows handling of the catheter without its contamination, but this packaging does not allow measurement of how much the catheter has been advanced in the blood vessel.